Digital D.C. motor speed control circuits are more efficient and therefore desirable for battery powered applications. Such circuits are usually frequency modulated or pulse-width modulated or a combination of both modulations.
Modulation is generally some function of the motor speed, which must be detected at the motor itself. A tachometer which senses the speed of the motor shaft as it rotates is often used. Another approach utilizes the fact that, when a motor is coasting (i.e. not driven) it operates as a generator which generates a back emf proportional to speed. Thus, the driving voltage can be applied to the motor when the back emf thereof crosses the triggering threshold of a back emf detector.
In prior art control circuits, however, at least one relatively large capacitor was or a plurality of smaller capacitors were required to assure stability of the feedback circuit or to average the output of a tachometer or both. The need for one or more capacitors has precluded complete adaptation of such circuits to integrated circuit technology. Moreover, if the capacitor fails, the resultant oscillation in such prior art circuits may damage the motor and other components of the circuit.